The present invention relates to a new and distinctive broccoli cultivar, designated 194-6-2CMS. There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include higher yield, resistance to diseases and insects, better stems and roots, tolerance to drought and heat, and better agronomic quality.
The cultivated plants associated with the species Brassica oleracea have been of great agricultural importance to mankind since ancient times. The introduction of hybrid cultivars to North America in the 1960's benefited the popularity of these crops by expanding the growing season, increasing yield and holding ability, and making large-scale production economically feasible. First generation (F.sub.1) hybrid cultivars possess the advantage of genetic uniformity without the inbreeding depression inherent in true-breeding lines. Developing commercial Brassica hybrids requires the development of homozygous inbred parent lines. Homozygous inbred lines of broccoli can be develop by self-pollinating (selfing) for 8 to 9 generations or by deriving doubled haploid plants from anther culture.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F.sub.1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location will be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for three or more years. The best lines are candidates for new commercial cultivars; those still deficient in a few traits may be used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from eight to 12 years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
There are two primary objectives for commercial heading broccoli breeding programs. These are yield and plant shape. Yield is determined in units of harvested boxes per acre. The stem thickness of a broccoli cultivar is a significant component of yield. Optimum plant shape is characterized by a cultivar that is suitable for three markets: exportation of plant crowns (crown cut), processing of florets for food service and frozen product and bunching of stalks and large side shoots for fresh market produce markets. A third objective can be the development of heat tolerant cultivars. Such cultivars would possess flower primordia that are less susceptible to damage from high temperatures. Such cultivars could be grown in conditions where traditional broccoli types produce damaged, unacceptable heads.
A most difficult task is the identification of individuals that are genetically superior, because for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of plant breeding is to develop new, unique and superior broccoli cultivars and hybrids. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing and mutations. The breeder has no direct control at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same broccoli traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic and soil conditions, and further selections are then made, during and at the end of the growing season. The cultivars which are developed are unpredictable. This unpredictability is because the breeder's selection occurs in unique environments, with no control at the DNA level (using conventional breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. The same breeder cannot produce the same cultivar twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of research monies to develop superior new broccoli cultivars.
The development of new broccoli cultivars requires the development and selection of broccoli varieties, the crossing of these varieties and selection of superior hybrid crosses. The hybrid seed is produced by manual crosses between selected male-fertile parents or by using male sterility systems. Additional data on parental lines, as well as the phenotype of the hybrid, influence the breeder's decision whether to continue with the specific hybrid cross.
Many Brassica species have a genetic characteristic of self-incompatibility. Self-incompatibility (SI) is a genetic system that favors outcrossing and therefore maximizes recombination and variability in a species. Such variability is desirable in nature for wide adaptation and species survival. Self-incompatibility has been the most common form of pollination control in F.sub.1 hybrid Brassica vegetables (Tsunoda et al., chapter 13). However, SI itself is not a satisfactory method for producing seed that is entirely or almost entirely hybrid. SI can breakdown at high temperatures and weakens at the end of a plant's reproductive stage. This can be extremely troublesome when the hybrid cross-pollination being produced uses parent lines that vary significantly. Problems such as achieving synchronous flowering between the two parent lines diminish the potential of SI to achieve high levels of hybridity. One example of such a cross-pollination is when cauliflower lines adapted to North American conditions are cross-pollinated with cauliflower lines adapted to tropical conditions.
Cytoplasmic male sterility (CMS) is another method used in Brassica vegetables species to produce F.sub.1 hybrids. This method of producing hybrids is a more recent development in Brassica compared to self-incompatibility. A genetic mutation contained in the cytoplasm, more specifically the mitochondrial DNA, is responsible for the lack of production of functional pollen. In Brassica, the cytoplasm has commonly been identified in and transferred from "Ogura"-type radish (Raphanus sativus) (Ogura, 1968). The major advantage of CMS over self-incompatibility is that under normal conditions, no pollen is produced in the female parent. This results in the production of 100% hybrid seed. Under certain stressful growth conditions, however, it may be possible to produce small amounts of fertile pollen in CMS plants. Brassica inbreds containing CMS are maintained by continued hybridization to their normal (fertile) counterpart inbred, commonly referred to as a "B" line or a maintainer line.
Pedigree breeding and recurrent selection breeding methods are used to develop cultivars from breeding populations. Breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools from which cultivars are developed by selfing and selection of desired phenotypes. The new cultivars are evaluated to determine which have commercial potential.
Pedigree breeding is used commonly for the improvement of crops. Two parents which possess favorable, complementary traits are crossed to produce an F.sub.1. An F.sub.2 population is produced by selfing one or several F.sub.1 's. Selection of the best individuals may begin in the F.sub.2 population; then, beginning in the F.sub.3, the best individuals in the best families are selected. Replicated testing of families can begin in the F.sub.4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F.sub.6 and F.sub.7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars.
Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified or created by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued.
Backcross breeding has been used to transfer genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or inbred line which is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
Descriptions of other breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., Allard, 1960; Simmonds, 1979; Sneep et al., 1979; Fehr, 1987).
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, the grower, processor and consumer; for special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.